The compound 2-keto-L-gulonic acid (2 KGA) is an important intermediate in the synthesis of ascorbic acid (Vitamin C). Numerous microorganisms are known to produce 2-keto-L-gulonic acid (2-KGA) from sorbitol or L-sorbose, for example, members of the genera Acetobacter and pseudomonas, (Japan Patent 40,154/1976). Numerous compounds have been proposed as intermediates in the production of 2-KGA from L-sorbose (Okazaki, et al. 1969. Agr. Biol. Chem. 33:207-211) the examples of this patent are concerned with the pathway through the intermediate L-sorbosone, and including the branch pathway to and from L-idonic acid; however, the principles involved in the invention apply to any pathway from L-sorbose to 2KGA. (Makover, et al. 1975, Biotechnol. and Bioeng. 17:1485-1514. ##STR1## The microbiological conversion of L-sorbose to L-sorbosone is carried out by the enzyme L-sorbose dehydrogenase which is produced by the microorganism used. In a microbiological process for producing 2-KGA, the conversion of L-sorbosone to 2-KGA is carried out by the enzyme L-sorbosone dehydrogenase produced by the microorganism (U.S. Pat. No. 3,907,639). Many microorganisms which produce L-sorbosone dehydrogenase do not produce the enzyme L-sorbose dehydrogenase.
In some microorganisms the yield of 2-KGA can be lowered because of the activity of 2-KGA reductase, an enzyme which reduces 2-KGA to L-idonic acid. Other pathways exist for the metabolism of L-sorbose by various enzymes. Thus pathways lower the yield of 2-KGA. It is desirable to eliminate, or minimize these pathways by eliminating, or minimizing the production of competing enzymes and thereby enhance the yield of 2-KGA. Since 2-KGA is not essential for bacterial growth, it is not possible to select directly for efficient producers. Currently known microbial producers of 2-KGA have been obtained by screening very large numbers of natural isolates. Guanglin, et al. 1980. Acta Microbiologica Sinica 20: 246-251. It is desirable to provide simplified methods for improving the production of 2-KGA.